1. Field of the Invention
The present invention relates to an ultrasonic transducer for a vehicle and a corresponding method for producing such an ultrasonic transducer.
2. Description of Related Art
Ultrasonic transducers are used in vehicles as parking assistance, for example. The parking assistance includes, for instance, a control device and ultrasound sensors that are provided with such ultrasonic transducers. For this purpose, in particular, transient characteristics of the ultrasonic transducer, for a so-called close range measuring capability, are a decisive functional requirement (for instance at <20 cm). An ultrasonic sensor has a housing into which the ultrasonic transducer is inserted. Such ultrasonic transducers are normally made up of a diaphragm pot and a transducer element situated inside of it. The diaphragm pot is usually formed or cut from a metallic material such as aluminum. It is coated with a primer, for reasons of protection from corrosion and enameling. The electromechanical transducer element (e.g. a piezo element) is applied to the bottom of the housing, for instance, adhered or bonded. The housing is filled with a damping material. One possibility for this is an injected silicone foam. These production steps are technically not trivial, for various reasons, and are therefore not able to be transferred to just any production sites. The chemical process of foaming, in particular, requires exact parameters, and is difficult to realize during manufacturing.
Instead of foaming the internal space of the housing, damping may be achieved by inserting foam parts into the housing. By doing that, it would be easy to replace the production step of foaming by a step that is simple to master. It has become apparent, however, that such sensors thereby acquire unfavorable transient characteristics compared to versions involving foaming, which impairs the decisive functional requirement for close range measuring capability. However, in particular the mechanical frictional connection of the damping material to the wall of the pot and to the diaphragm, over aging and temperature, has to be ensured. This is achievable in the case of insertion parts only by using oversized parts and inserting them in a prestressed position, which means expenditure in production and housing construction which has to absorb the stress. Consequently, a frictional connection by an exact-shape filling is more favorably achievable using liquid materials which harden afterwards.
By measuring the spectral components of the transient processes, it has turned out that a substantial proportion is based on resonances outside the working frequency (48 kHz) of the ultrasonic sensor, in particular, components around 30 kHz and 70 kHz being significant. These are caused by tilting motion and creasing motion operations in the housing wall. Up to now, a remedial measure against this was carried out by thickening the diaphragm pot by putting in a bead at the upper edge of the wall, whereby the wall of the pot is stiffened, but the vibration modes that are excited are not damped. With that, the vibrations at the edge of the pot, or rather the bead, are reflected by faulty adaptation of the mechanical impedance, but they are not converted into thermal energy, whereby damping would occur.
Moreover, for the damping of the housing wall vibrations, housing filling materials are used which are made up of damping materials and/or additionally include such materials. A material such as a strongly damping foaming material, which fills up the inner space of the diaphragm pot with an almost homogeneous consistency, thereby brings about not only great damping of the wall vibrations, but also of the useful vibrations of the diaphragm pot's diaphragm. This leads to a low electroacoustical efficiency of the ultrasonic transducer. The driving electronics, that is, the transmitting current source and the transformer, enter into saturation effects at too great a damping, which additionally reduce the bandwidth of the tuned system transducer/circuit elements.
A further remedial measure is an external decoupling ring that is applied to the wall of the diaphragm pot. It can be shown by measuring technology that, in the case of mass production transducer construction, the wall modes are then almost completely suppressed. The mechanical contact of this decoupling ring with the outer wall of the diaphragm pot is, however, exposed to outer environmental influences, such as soiling, tilting of the sensor in its support or penetrating water.
Published German patent document DE 199 12 772 describes an ultrasonic sensor for measuring separation distances, a diaphragm pot being equipped with an elliptical and circular recess over a transducer element having a plurality of insulators that are layered one over another and are mechanically held in the diaphragm pot by a bore guard ring in an annular groove. A connection to the inner wall is not described. These insulators, for example, are made up of different damping materials such as a silicone disc, felt and cork. The insulators have the shape of the elliptical recess, and completely cover the transducer element.
Published German patent document DE 199 17 862 describes an ultrasonic sensor whose piezoelectric transducer element is covered by a sound-absorbing material. This may be felt, for example. An insulating resin, such as silicone rubber or urethane rubber, seals the piezoelectric transducer element and the sound-absorbing material. The inner space of the diaphragm pot has an elliptical and circular contour. The transducer element is fastened on the diaphragm and surrounded by a ring section which may, for instance, have a thinner wall thickness than the section onto which the transducer element is mounted. In one embodiment, the transducer element and the ring section are completely covered by the sound-absorbing material. In another embodiment, the ring section of the diaphragm surrounding the transducer element is covered by an insulating resin having a very low layer thickness. This may be silicone resin or urethane resin, for example. Hereby the undesired vibration of the ring section is damped by a thinner wall thickness. A connection of this thin damping layer of the insulating resin to the inner wall of the diaphragm pot is not described.
An ultrasonic sensor according to published German patent document DE 103 41 900 has an inner space of a diaphragm pot which, above a transducer element, is filled up in layers with felt and above that with silicone rubber. In the silicone rubber a section is situated made of silicone foam having bubbles. The silicone rubber seals off the diaphragm pot. The felt layer completely covers the transducer element and is completely covered by the silicone rubber.